how much can you do with prop options

[limitout]

after reading the endless threads here about props to tweak top speeds I was wondering just how much room is there for changing you boats attitude with a new prop?

I have a jon boat with the recommended prop that came with it and it runs the recommended rpms but seams a little on the slow side for speed.

it would stand straight up on take off (literally the transom was within an inch of going under)

I always "assumed" the recommended prop listed for my motor was right and only finding the pitch was needed but it looks like there is a whole lot more to picking a prop then just whats listed for your motor so how much room is there a way to get more speed out of it and change how it acts?

EDIT:
its a Suzuki df40 on a 15 ft flatboat with this dealer recommended prop on it:
POWER TECH SRD3 PROPELLER 11.25" DIAMETER, 13 PITCH, 3 BLADES, RIGHT-HAND ROTATION, BEST ALL PURPOSE, SS PROP, 20 DEGREES RAKE - POLISHED FINISH ONLY

It can be seen here: http://www.ptprop.com/index.php?pag...facturer_id=0&option=com_virtuemart&Itemid=46

 

[SkiDad]

Re: how much can you do with prop options

if you are getting 30 mph i think is good for 40 hp in that boat. My calculations put you at 25-26 mph with a 12 pitch prop. You might gain 2-3 mph with a higher pitch (14p or 15p) but you will not get 10 mph. if you have a smart phone you can get a free app to find out your speed.

it take a lot of power to break 30 mph barrier

 

[limitout]

Re: how much can you do with prop options

if you are getting 30 mph i think is good for 40 hp in that boat. My calculations put you at 25-26 mph with a 12 pitch prop. You might gain 2-3 mph with a higher pitch (14p or 15p) but you will not get 10 mph. if you have a smart phone you can get a free app to find out your speed.

it take a lot of power to break 30 mph barrier

25-26mph definitely sounds about right, I was being very optimistic guessing my speed was 25-30mph lol.

it would be worth it to me to get that extra 3mph but if I go to a higher pitch how do I know how far to drop the wheel size down to keep the rpms near 5700-5800 rpm?

im a lifelong boater of over 50 years but as you can tell I never had to size a prop before, I always relied on the dealer to set it up for me.

I have no smart phone, in fact im so old school I don't even have a cell phone lol, I will have to rely on the calculations

where do I find the formula to calculate the correct prop I need to get the maximum speed at the correct rpms?

 

[smokeonthewater]

Re: how much can you do with prop options

your you aren't going to get anywhere unless you can figure out (for sure) what prop you currently have (there should be markings on it) and unless you come up with some measure of speed you can't compare results... a friend with a smart phone, the gps out of a car... elapsed time over a measured distance IE 1/4 mile in 30 seconds is 30 mph (heck flag down another boat and ask them to pace you... the formula of which you speak is only to get a starting point... it may well be worse than what you currently have.. fine tuning is done by measuring current performance and adjusting to make it better

Also there are other places to look for max speed such as condition of the hull. if it's beat up it'll hurt you and if the motor were running rich or late timing it could be down on top end power without blowing up like lean or early timing would do.

Are you trimming up for max speed? Do you have the correct shaft length engine for your boat and set at the right height on the transom.. the anti vent plate, when parallel with the bottom, should be just a little higher than the bottom.

 

[SkiDad]

Re: how much can you do with prop options

if you have a 12p to increase speed you need about a 14p - this will cause your RPM will go down ~400 - that is just how it works - as long as you don't drop below 5000 you should be good. but I would probably suggest staying closer to 5200 rpm to keep your takeoff strong. I use this site: Propeller Calculator / Prop Calculator - i put in 2.27 for your ratio and about 10% for slip.

Smokeonthewater is right you need to get that prop off and get the markings on it - if it is stainless it will have a pitch for sure. If you like the prop you have getting another in a larger pitch will give a predicable results.

 

[smokeonthewater]

Re: how much can you do with prop options

generally the engines are rated for an rpm range IE 5300-5800 you want to prop so that you run near the top lightly loaded and near the bottom fully loaded

a higher pitch prop will only raise speed if you have power to spare... as you are not hitting your top rpm, you do not.

a lower pitch prop would give you more speed by raising your rpm's if you were lugging the engine below it's peak powerband but your current rpm's indicate that that is not the case....

kinda like climbing a mountain in a 5 speed car... currently you are on the floor at almost redline in 3rd gear. You can downshift to second (smaller pitch) but you then over rev and lose speed or the engine blows, or you can upshift to 4th (higher pitch) but then you bog the engine and lose speed....

Unless you can find more power in the engine, reduce drag from the boat or find a problem, you are pretty much spot on with your prop.

Going with the car analogy, You COULD gain speed by upshifting but to do so you would need to increase horsepower or reduce weight/drag to be able to pull the higher gear

 

[limitout]

Re: how much can you do with prop options

your you aren't going to get anywhere unless you can figure out (for sure) what prop you currently have (there should be markings on it)

I confirmed its a:
POWER TECH 11.25" DIAMETER, 13 PITCH, 3 BLADES, RIGHT-HAND ROTATION, BEST ALL PURPOSE, PROP, 20 DEGREES RAKE - POLISHED FINISH ONLY

I will add this info to the first post

 

[limitout]

Re: how much can you do with prop options

I was thinking if rpms dropped with the 15 pitch and I went from a 11 1/4" wheel to a 10 1/2" wheel it might have close to the same rpms but give me more speed? how much does decreasing wheel size raise rpm's?

maybe im not thinking about this the right way but I was thinking higher pitch + smaller diameter wheel = a net balance to keep rpm's pretty much the same while increasing speed.

in other words cant different prop combinations of pitch and wheel sizes give you the same correct rpms but yet yield different top end speeds?

I see props of the same pitch listed here and one is 10" diameter and 10 1/2" diameter, and 11" diameter so doesnt the 10" prop have to turn higher rpms then 10 1/2 or the 11"

I keep hearing going up or down 2 numbers in pitch = 400 rpms but what does changing one inch in diameter = to in higher or lower rpms if the pitch stays the same? is there a rule of thumb for changing diameter sizes?

 

[steelespike]

Re: how much can you do with prop options

You can't diddle with the prop diameter to force the rpm.It will affect slip,hole shot and may cause ventilation(cavitation) problems.
Rule of thumb you will lose about 150-200 rpm per inch increase in pitch. A 14" prop should act like a 14" prop.
In fact if you check prop sizes you may notice that as pitch goes up diameter goes down.
As I understand it you can add a gps app to a cell phone. Doesn't your fishing buddy have a cell phone/or maybe a nephew/niece,
neighbor Boat mechanic,bowling buddy, you get the idea.
There are a few little tweaks that might help.

 

[smokeonthewater]

Re: how much can you do with prop options

prop diameter is a function of horsepower prop shaft rpm and to some extent the size of the boat but that is generally on a out of the norm application such as a 10 hp motor for a 30' sailboat or a 400 hp motor in a 16' drag boat...

For your (very normal) application you prop by selecting pitch based on rpm and the diameter is what it is...

changing rpm slightly will affect holeshot and low speed cruising MUCH more than top speed and changing it enough to affect top speed will totally screw up holeshot and low speed... If you decrease it enough to allow more pitch at top speed your slip numbers will be through the roof, your speed will decrease, and your mpg's will fall

You are not going to gain 5 mph changing props when you are as close as you are... 0.5-1.5 maybe... I would consider removing or if possible adjusting the tabs for a test but we still need to measure your speed somehow tho If you adjust your hull IE tabs or weight distribution n trim and see an increase in rpm you can be pretty sure you gained speed

 

[hwsiii]

Re: how much can you do with prop options

limitout, " I was thinking if rpms dropped with the 15 pitch and I went from a 11 1/4" wheel to a 10 1/2" wheel it might have close to the same rpms but give me more speed? how much does decreasing wheel size raise rpm's?

maybe im not thinking about this the right way but I was thinking higher pitch + smaller diameter wheel = a net balance to keep rpm's pretty much the same while increasing speed.

in other words cant different prop combinations of pitch and wheel sizes give you the same correct rpms but yet yield different top end speeds? "

In your particular case you definitely have the wrong prop for your boat and motor combination, the high degree of rake in that prop required you to use the trim tabs and with that hull that much rake and blade surface area is wasting a lot of HP and Torque from the motor and thus speed. The information you have received thus far is the normal in almost all cases, but in my opinion in this particular case, because of the prop you have now you can gain the extra speed you are looking for by changing to another prop, even an aluminum one can make it go faster.

But, without having some actual RPM and GPS speed numbers it will make it harder to find a better prop.

The theory that you have postulated can and does work, but prop geometry is what makes it work, and that theory does include prop diameter. As a normal course of events prop diameter has a very big relationship to blade surface area which also includes the amount of thrust and drag from any particular prop, and prop diameter is definitely a part of blade geometry. Spike and smoke both understand the principles as they said, spike mentioned " In fact if you check prop sizes you may notice that as pitch goes up diameter goes down. " And smoke said " prop diameter is a function of horsepower prop shaft rpm and to some extent the size of the boat ". The difference that matters in this case, in my opinion, is the blade geometry of the prop that you are using now, as it has a much different blade geometry than what is needed for your boat.

BUT, without GPS speed and RPM's you are just assuming a lot of the information you have posted on speed, and that does NOT help Anyone in picking a better prop for your boat and motor.

And your calculations are wrong " based on the calculator, 5700 rpm's at 2.27 gear ratio with 15 pitch prop and a slip of 10% says I would get 30mph or am I confused about how it works? "

It is 16% slip.


H

 

[limitout]

Re: how much can you do with prop options

You can't diddle with the prop diameter to force the rpm.It will affect slip,hole shot and may cause ventilation(cavitation) problems.

prop diameter is a function of horsepower prop shaft rpm and to some extent the size of the boat but that is generally on a out of the norm application such as a 10 hp motor for a 30' sailboat or a 400 hp motor in a 16' drag boat...

For your (very normal) application you prop by selecting pitch based on rpm and the diameter is what it is...

maybe I didn't explain my thinking the right way. props are available in different diameters with the same pitch as seen here:
15” Pitch Suzuki Outboard Boat Propellers 40 HP 1999 - 2010 DF 40, 4-Stroke - iboats

I can select from 10", 10 1/2" or 11" diameter ss props depending purely on the brand I choose and they would all be in the 15 pitch but there is no standard "it is what it is" size for my application that I can see or they would/should all list the same diameter size for my 15 pitch prop with other sizes available for those "special" applications you mentioned so it is confusing me because changing diameter should have as big an affect if not bigger then changing the pitch (as I understand it).

that said if i go to a larger pitch, based on the rule of thumb, I will lose roughly 400 rpms and that's where I came to the second part of my thinking process which is, now that I chose to go with a 15 pitch prop and I lost 400 rpms (in theory) so should I also go down to a lower diameter size because the smaller diamer size will increase the rpms the motor will achieve with the 15 pitch prop and "in theory" the end result will increase the rpms and hopefully bring them back to near the 5800 rpm max the motor recommends to have.

I was only thinking to reduce wheel diameter to maintain/restore the desired rpms and not to gain hp, top end speed, torque, or hole shot because (I think) that that's only done thru the pitch.

thanks for all your help with this guys, this stuff gets confusing

 

[limitout]

Re: how much can you do with prop options

In your particular case you definitely have the wrong prop for your boat and motor combination, the high degree of rake in that prop required you to use the trim tabs and with that hull that much rake and blade surface area is wasting a lot of HP and Torque from the motor and thus speed. The information you have received thus far is the normal in almost all cases, but in my opinion in this particular case, because of the prop you have now you can gain the extra speed you are looking for by changing to another prop, even an aluminum one can make it go faster.

But, without having some actual RPM and GPS speed numbers it will make it harder to find a better prop.

The theory that you have postulated can and does work, but prop geometry is what makes it work, and that theory does include prop diameter. As a normal course of events prop diameter has a very big relationship to blade surface area which also includes the amount of thrust and drag from any particular prop, and prop diameter is definitely a part of blade geometry. Spike and smoke both understand the principles as they said, spike mentioned " In fact if you check prop sizes you may notice that as pitch goes up diameter goes down. " And smoke said " prop diameter is a function of horsepower prop shaft rpm and to some extent the size of the boat ". The difference that matters in this case, in my opinion, is the blade geometry of the prop that you are using now, as it has a much different blade geometry than what is needed for your boat.

BUT, without GPS speed and RPM's you are just assuming a lot of the information you have posted on speed, and that does NOT help Anyone in picking a better prop for your boat and motor.

And your calculations are wrong " based on the calculator, 5700 rpm's at 2.27 gear ratio with 15 pitch prop and a slip of 10% says I would get 30mph or am I confused about how it works? "

View attachment 215286 It is 16% slip.


H

so the dealer probably past off a prop he couldn't sell hoping I didn't know the difference, which I didn't

what "should" the prop "rake" be for an average flatboat? so I know what I should be looking for

also I don't see the "rake" specified or highlighted anywhere in the prop details they list so it never entered my mind as important. can you elaborate more about how rake affects things?

how easy would it be to sell this ss prop and what would be the correct application for it to be used on so I know who to target to sell it to.

 

[hwsiii]

Re: how much can you do with prop options

limitout, I would not blame the dealer for this problem, as the prop does reach the correct RPM's and many dealers don't understand blade geometry any better than you do. Their real concern is that the motor does not fall below or above the recommended factory RPM's.

Most aluminum flat bottom skiffs need no rake in their props, because of their light weight and flat bottom (very little surface area), increasing rake can cause porpoising as the bow rises from either wind getting up under it or even very small waves lifting the bow and also load distribution.

Progressive Rake The most Bow lift available with good speed, great holding power at high speeds and Rough Water and less cavitation and ventilation

Flat Rake Has Great Bow lift and not quite as much holding power or Bow lift as Progressive Rake

0 Rake Best prop for Stern lift and produces the most power, it is the best prop for boats that have a tendency to porpoise

Because high deadrise hulls have much more surface area, and normally much higher weights as well, usually the less surface area in the water the less drag that the boat produces and thus higher speeds. Rake, and the different types of rake there are in a prop is a very complicated subject, so I am just touching on the basics here.

I think you will find it very hard to sell the prop the way it is, but I do think that prop is deigned for boats with much more deadrise and not flat bottom aluminum hulls.

H

 

[smokeonthewater]

Re: how much can you do with prop options

didn't catch that it had a lot of rake so yeah wrong prop


the different diameter of the different props you have looked at are functions of the different blade geometry and are decided by the manufacturer to achieve the blade surface area... in fact by selecting a prop based solely on diameter you could get exactly the opposite effect of what you are looking for....

For now, focus on finding us the info we need IE mph... get us mph at various rpm's ie xx mph at 3500, 4000, 4500, etc,, work with trim at each throttle setting to get the best speed

if the tabs are adjustable.. at some point try bringing em up above the bottom... even parallel with the bottom they still have an effect.... If your tm is on the stern consider moving it to the bow.... more weight forward will very possibly help you... portable tanks" one or more could possibly go forward

these are all things you can play with OR you can leave em all as they are and just work on getting the best prop you can for how everything is now... up to you

I put a 90 4 stroke on my dad's 18.5 bass tracker flat bottom and I moved the on board charger. 2 tm batts and the start batt up front... it already had the tm up front and an anchor winch on the bow w a good size anchor....... Eventually I ended up moving the start batt to the stern because the bow sat just a little low with one person in the boat fishing from the bow but the boat ran better and better the more weight in the front... it would pick up almost 2 mph by me (250 lbs) moving from the stern to the bow underway and dad trimming up to compensate

You are in good hands here and when all numbers in you'll likely leave here with the exact prop to go buy and be thrilled with...

do you still HAVE the alum prop it came with? if so test w that one too

BTW on my local C/L there were several used car gps units in the $20-$30 range... just about all of em will tell you mph

 

[Frank Acampora]

Re: how much can you do with prop options

Well, this is a prop forum, but no one has mentioned engine position on the transom. Suzys have a slightly longer mid/lower unit and the anti-ventilation plate (cavitation plate) on many boats will be an inch or more below the bottom of the hull. Simply raising the engine one or two holes so the anti-vent plate is even with the bottom or slightly above the bottom will increase speed. You raise the engine until it runs normally on the straight and ventilates a little in sharp turns and that is approximately correct.

 

[limitout]

Re: how much can you do with prop options

Well, this is a prop forum, but no one has mentioned engine position on the transom. Suzys have a slightly longer mid/lower unit and the anti-ventilation plate (cavitation plate) on many boats will be an inch or more below the bottom of the hull. Simply raising the engine one or two holes so the anti-vent plate is even with the bottom or slightly above the bottom will increase speed. You raise the engine until it runs normally on the straight and ventilates a little in sharp turns and that is approximately correct.

smoke already covered that but its a good reminder for you to bring up.

 

[limitout]

Re: how much can you do with prop options

I do think that prop is deigned for boats with much more deadrise and not flat bottom aluminum hulls.

H

ok so the dealer wasn't likely trying to get one past me, that's good to know

can you clarify what that means in simple terms? do you mean its better for big heavy boats or boats with a lot of drag like pontoons?

 

[limitout]

Re: how much can you do with prop options

didn't catch that it had a lot of rake so yeah wrong prop
well that sux lol, im now mad at myself for not listening to the voices in my head telling me somethings not right and it should be faster

the different diameter of the different props you have looked at are functions of the different blade geometry and are decided by the manufacturer to achieve the blade surface area... in fact by selecting a prop based solely on diameter you could get exactly the opposite effect of what you are looking for....
well when im given a choice I keep asking why until I understand why there even is a choice so that I can better make the correct choice and with props there is a lot of differences which aren't explained such as why 4 different prop makers all have the "supposed" prop for my motor all in 4 different sizes with no explaination why. I know wheel size will affect things a lot with the ratios but it seams its not mentioned anywhere like its nothing to pay attention to

For now, focus on finding us the info we need IE mph... get us mph at various rpm's ie xx mph at 3500, 4000, 4500, etc,, work with trim at each throttle setting to get the best speed

if the tabs are adjustable.. at some point try bringing em up above the bottom... even parallel with the bottom they still have an effect....
as I mentioned several times already the trim tabs are already adjusted completely up out of the flow of water up slightly above parallel with the bottom about an 1/8" or so and they apply no force or drag when on a plane

If your tm is on the stern consider moving it to the bow.... more weight forward will very possibly help you... portable tanks" one or more could possibly go forward

these are all things you can play with OR you can leave em all as they are and just work on getting the best prop you can for how everything is now... up to you

I put a 90 4 stroke on my dad's 18.5 bass tracker flat bottom and I moved the on board charger. 2 tm batts and the start batt up front... it already had the tm up front and an anchor winch on the bow w a good size anchor....... Eventually I ended up moving the start batt to the stern because the bow sat just a little low with one person in the boat fishing from the bow but the boat ran better and better the more weight in the front... it would pick up almost 2 mph by me (250 lbs) moving from the stern to the bow underway and dad trimming up to compensate
yep, I did the same sort of rearranging of things with mine and I have two tanks but only fill one to reduce weight in the rear

You are in good hands here and when all numbers in you'll likely leave here with the exact prop to go buy and be thrilled with... yep, im not really looking to be the fastest thing on water or be able to race other boats but I do want the full speed potential I can get out of it, whatever that might be. just thinking I never needed tabs at all and if I just the right prop in the first place it would have solved my planning issues

do you still HAVE the alum prop it came with? if so test w that one too
no. unfortunately I sold it the other day on ebay so its gone. it was a 11 5/8" x 12 pitch

BTW on my local C/L there were several used car gps units in the $20-$30 range... just about all of em will tell you mph

so my boat is acting the way it is because I have a 20* rake prop and I think you guys are saying I need a 0* rake prop for it to act right?

if so then shouldn't I be doing my testing with a different rake prop instead of the one I have?

im thinking the local dealer should be able to give me some loaners to test out to find the right one if I bought it from him, least that's how they used to do it last time I bought a prop about 10 years ago

 

[smokeonthewater]

Re: how much can you do with prop options

heck yes... if you can get loaners that is perfect..... around here you have to buy em and as long as they don't get one single mark on em you can exchange..... I would say go ahead and test with the current prop 1: because you already have it so it costs you nothing except a few minutes of your time, and 2: so that you (and we) will know what and how much the improvement is.....